Selective Recovery of Copper from Municipal Solid Waste Incineration Bottom Ash

A hydrometallurgical process was investigated as a method to selectively recover copper from municipal solid waste incineration (MSWI) bottom ash. In Norway, bottom ash is mainly disposed of instead of being utilized. When landfilled, hazardous components, such as copper and zinc, can potentially be released to the environment and cause groundwater pollution. Therefore, extraction of copper and zinc would be a favorable solution for both disposal and utilization of bottom ash, as well as a potential source for secondary materials. In order to be economically feasible to recover metals from bottom ash, other metals and components probably need to be recovered as well. Acidic leaching solutions are most commonly used, however, alkaline solutions are generally more selective, less corrosive and have a lower reagent consumption according to previous studies. Ammonia-ammonium carbonate (AAC) is stated to be a suitable choice of leach solution for selective recovery of copper and zinc. The different substances of the AAC solution can be recovered, recombined and used in subsequent processing. In the present study, AAC leaching of bottom ash followed by a cementation procedure in order to selectively precipitate copper was investigated. Co-dissolution of zinc was also monitored during the study due to possible subsequent recovery based on procedures established in previous studies. MSWI bottom ash originating from different incineration plants in Norway was characterized and used in the study. The bottom ash was leached with an aqueous solution of ammonia (NH3) and ammonium carbonate ((NH4)2CO3). In order to efficiently extract copper from bottom ash, the AAC leaching procedure was optimized in terms of particle size, total ammonia concentration, leaching temperature, pulp density (solid/liquid ratio), and leaching time. The optimal leaching conditions were found to be 4.7 M, 60 C, 100 g bottom ash per liter leach solution and 180 minutes of leaching time, before the liquid and solid phases were separated. The pregnant leach solution (PLS) was quantitatively analyzed with inductively coupled plasma optical emission spectroscopy (ICP-OES). Extraction of more than 98 % of the copper from bottom ash was achieved. The copper was subsequently precipitated from the PLS by a cementation procedure with metallic zinc dust. After cementation, the PLS was quantitatively analyzed with ICP-OES and the solid residue was qualitatively analyzed using X-ray diffraction. More than 90% of copper was recovered from the PLS by cementation.